Because a titanium plate is excellent in corrosion resistance, it is used widely for members for a heat exchanger of a chemical plant, a power plant, a food processing plant, and the like, consumer products such as a camera body and a kitchen instrument, members for transportation equipment such as a motorcycle and an automobile, and exterior material for household electrical appliances and the like.
Among them, with respect to the plate type heat exchanger, in order to improve the heat exchange efficiency, it is necessary to work the titanium plate into a corrugated shape by press-forming, and to increase the surface area. Therefore, when a titanium plate is to be applied to a plate type heat exchanger, excellent formability is required for the titanium plate.
Also, when a titanium plate is to be applied to a plate type heat exchanger, in addition to the formability described above, strength of a specific level or more is also required for the titanium plate in order to achieve improvement of the durability and weight reduction required as the plate type heat exchanger.
Here, the titanium plate (pure titanium for industrial use) is specified in the standards of JIS H 4600, and is classified to the grades of JIS Type 1, Type 2, Type 3, and the like according to the content of Fe, O, and the like, the strength, and so on. As the grade becomes higher, the content of Fe, O, and the like increases and the strength becomes higher, and therefore, when a titanium plate is to be used for the use requiring a high strength, those with a high grade are used. In contrast, a titanium plate with a low grade namely the titanium plate of JIS Type 1 for example has less content of Fe, O, and the like, and the ductility becomes high (the formability improves). Therefore, when a titanium plate is to be used for a use requiring excellent formability, those of JIS Type 1 are used.
However, when the content of Fe, O, and the like is increased and the strength of the titanium plate is improved, the formability deteriorates, whereas when the content of Fe, O, and the like is reduced and the formability of the titanium plate is improved, the strength deteriorates.
Also, as a method for improving the strength of a titanium plate, there also exists a method of refining the grains of the titanium plate, however, the formability of the titanium plate deteriorates accompanying refinement of the grains.
As described above, when a titanium plate is to be applied to a plate type heat exchanger, there is a fact that the strength of a specific level or more (the strength of JIS Type 2, Type 3) and excellent formability are required for the titanium plate. However, it was very difficult to improve the formability while avoiding deterioration of the strength.
Therefore, in the past, with respect to a titanium plate, various technologies as described below focusing on improvement of the strength and the formability have been disclosed.
For example, in Patent Literature 1, there is disclosed a method for manufacturing a titanium plate excellent in formability which has the crystal structure of a hexagonal system and contains a predetermined amount of H, O, N, and Fe with the remainder consisting of Ti and inevitable impurities, and in which the Kearns factor f value defined by a predetermined formula is 0.60 or more.
In Patent Literature 2, a titanium plate excellent in bendability and stretch formability is disclosed which contains a predetermined amount of Fe and O with the remainder consisting of Ti and inevitable impurities and has an equi-axed α+β2 phase microstructure, and in which the angle between the direction showing the peak of the (0001) pole figure of the α phase and the normal direction of the rolling direction is 40° or more.
In Patent Literature 3, a titanium plate having a high strength and excellent in formability is disclosed which contains the β-stabilizing element such as Fe as well as O of a predetermined amount with the remainder consisting of Ti and inevitable impurities, and in which the area ratio of the α phase whose average value of the angle between the normal direction of the (0001) plane of the α phase and the normal direction of the rolling plane is 60° or less and the angle is 70° or more relative to the entire α phase is 30% or less.
In Patent Literature 4, a titanium plate having a high strength and excellent in deep drawability is disclosed which contains the β-stabilizing element such as Fe as well as O of a predetermined amount with the remainder consisting of Ti and inevitable impurities, and in which the area ratio of the α phase whose average value of the inclination angle between the normal direction of the (0001) plane of the α phase and the normal direction of the rolling plane is 45° or less and the inclination angle is 50° or more relative to the entire α phase is 10% or less.